Traditionally, there are two processes for the production of special pulps having a high content of alpha cellulose, such as dissolving pulp: the far-advanced acidic bisulphite cooking and the prehydrolysis-sulphate cooking. The former was developed at the beginning of the 20th century and the latter in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, p. 649 to 672, Interscience Publishers, New York, 1968. The basic idea in both processes is to remove as much hemicellulose as possible from cellulose fibres in connection with the delignification so as to obtain a high content of alpha cellulose. This is essential because the various uses of dissolving pulp, for instance, do not tolerate short-chained hemicellulose molecules with indefinite structure. In the sulphite process, the removal of hemicellulose takes place during the cooking simultaneously with the dissolving of lignin. The cooking conditions are highly acidic and the temperature varies from 140.degree. to 150.degree. C., whereby the hydrolysis is strong. The result, however, is always a compromise with delignification, and no high content of alpha cellulose is obtained. Another drawback is the decrease in the degree of polymerization of cellulose and the yield losses, which also limit the hydrolysis possibilities. Various improvements have been suggested in traditional sulphite cooking, the use of additional chemicals, for instance. Such additional chemicals, used in addition to the basic chemicals of sulphite cooking, include sulphide, white liquor, and anthraquinone, see e.g. Finnish Patent Specification 67 104 and U.S. Pat. No. 4,213,821. These sulphite cooking variations do not, however, imply hydrolytic conditions.
A separate prehydrolysis step is interesting in the view of the fact that it enables the adjustment of the hydrolysis of hemicelluloses as desired by varying the hydrolysis conditions. In the prehydrolysis-sulphate process the delignification is not carried out until in a separate second cooking step. The prehydrolysis is carried out either as a water prehydrolysis or in the presence of a catalyst. Organic acids liberated from wood in the water prehydrolysis perform a major part of the process, whereas small amounts of mineral acid or sulphur dioxide, in some cases even sulphite waste liquor, are added to the digester in "assisted" prehydrolysis. It has previously been necessary to effect the lignin dissolving step after the prehydrolysis as sulphate cooking which has several drawbacks. The prehydrolysis-sulphate process has e.g. the following drawbacks:
The yield is low because of the strong alkaline reaction conditions which cause splitting of cellulose. Thus the wood consumption per one ton of cellulose is high.
The content of residual lignin is rather high because the step for the removal of residual lignin in the sulphate cooking process is extremely non-selective. Thus there is a great need of bleaching for complete removal of lignin, and the consumption of chemicals is high; further, at least five bleaching steps are required.
Industrial realization of sulphate cooking is complicated, and the cost of investment very high.
Previously the use of sulphite cooking has not been possible, because it is not possible to dissolve from wood material lignin deactivated in the prehydrolysis by means of traditional sulphite cooking processes. It has been regarded as impossible to use a sulphite cooking step (cf. Rydholm above) even though it would have advantages over sulphate cooking.